1. Field of the Invention
The present invention relates to a transmitting and receiving apparatus for data communications using a spread spectrum signal.
2. Description of the Prior Art
Spread spectrum communications has been identified as a suitable method for local area wireless data communications systems (e.g., wireless LANs) and power-line carrier (PLC) data communications because of its good transmission characteristics in multipath environments and excellent ability to eliminate interference signals. The major frequency band allocated to wireless LAN systems is the ISM band used by industry, science, and medicine. The ISM band is the frequency band used by devices using electromagnetic power waves, such as microwave ovens, and transmitters and receivers used in wireless LAN systems must be able to maintain normal data communications even under extremely high interference conditions.
A variety of methods not requiring spreading signal synchronization has been proposed as a means of simplifying the data receiver in spread spectrum communications. One of these methods is the spread spectrum differential detection method whereby the period of the spreading signal is synchronized to the data symbol period. An example of this method is described in Japanese patent laid-open number 1987-257224. The configuration and operation of a spread spectrum communications apparatus applying this spread spectrum differential detection method is described below with reference to accompanying figures.
FIG. 26 is a block diagram of a spread spectrum differential detection method transmitter and receiver. FIGS. 27a and 27b show waveform diagrams of the signals processed by various transmitter and receiver components. As shown in FIG. 26, the transmitter 10 comprises a differential encoder 11, PSK (phase shift keying) modulator 12, spreading signal generator 13, multiplier 14, and symbol clock generator 15. The clock generator 15 supplies the symbol clock CK of period T to the differential encoder 11, PSK modulator 12, and spreading signal generator 13.
The receiver 20' comprises a differential detector 22 and a decoder 23. The differential detector 22 further comprises a delay 221, multiplier 222, and low-pass filter (LPF) 223.
The bit stream data (binary data of value .+-.1) is read synchronized to the symbol clock CK, and is differential coded by the differential encoder 11. The PSK modulator 12 modulates the carrier wave with the differential coded data to obtain data modulated signal p, which is a binary PSK signal of symbol cycle period T. As a result, the data modulated signal p is the same phase as the previous symbol when data d is 1, and is opposite phase to the previous symbol when data d is -1. The spreading signal generator 13 generates the spreading signal q synchronized to and with the same period as the symbol clock CK. The spreading signal q is a constant amplitude, pseudorandom pulse wave generated from pseudorandom series. The multiplier 14 multiplies the data modulated signal p and spreading signal q to output the spread spectrum signal a.
FIG. 27a shows the time-based waveforms of the data modulated signal p, spreading signal q, and spread spectrum signal a. The baseband waves of the data modulated signal p and spread spectrum signal a are shown for convenience.
The spread spectrum signal a thus obtained is input through the transmission path to the receiver 20'. The differential detector 22 multiplies the received spread spectrum signal a by delayed signal a.sub.d (which is the spread spectrum signal a delayed symbol cycle period T by the delay 221) using the multiplier 222, and removes the high frequency component of the product using the LPF 223 to obtain detector output c. Because multiplying spreading signal components will always result in a constant value, only the data modulation component will appear in the detector output c. As with the differential detection output to the normal differential PSK signal, the detector output c is therefore a positive value when there is no phase change from the previous symbol, and is a negative value when opposite phase to the previous symbol. The decoder 23 outputs the decoded data d' as a value of +1 when the detection output c is positive, and -1 when negative.
FIG. 27b shows the time-based waveforms of the received spread spectrum signal a, delayed signal a.sub.d, and detector output c. As in FIG. 27a, the baseband waves of the spread spectrum signal a and delayed signal a.sub.d are shown. It is to be noted that the normally received spread spectrum signal a has jamming, interference, or distortion components added in the transmission path. The effects of such jamming, etc., are removed from the waveforms shown in FIG. 27b.
By means of this configuration, a transmitter and receiver of relatively simple construction not requiring complex means for spreading signal synchronization and other functions can be obtained while retaining the jamming elimination capability and multipath transmission performance characteristic of spread spectrum communications.
When extremely strong interference components are added to the spread spectrum signal band, however, this transmitter and receiver is incapable of signal reception when the band of the interference component overlaps only part of the signal band. In addition, a wide band delay having a constant delay characteristic across the complete spread spectrum signal band must be used, and such delays are difficult to achieve.